There are many attractions in using non-contact near-IR sources, in particular diode and fibre lasers, to generate images from coatings for applications such as variable information packaging. Favourable attributes of diode and fibre lasers such as economy, portability and ease of use, are attractive for current needs in the packaging industry, such as in-store labelling.
Virtually all colour change materials used in laser sensitive coatings absorb mid-IR radiation and therefore can be activated using mid-IR lasers such as CO2 lasers that emit radiation at approximately 10.6 microns. However, these same colour change materials usually have negligible absorbance in the near-IR region and consequently they cannot be activated using near-IR lasers that emit in the 700 to 2,500 nm region. Near-IR lasers are often more convenient to use than CO2 lasers particularly in term of retrofitting to production lines due to their more compact size. Also near-IR lasers are more readily made into an individually addressed ‘array’ type emitting device than CO2 lasers are, which due to the absence of moving galvo mirrors offers the potential to image at fast line speeds.
However, by incorporating, into these same ink formulations, materials which absorb radiation from near-IR sources such as diode and fibre lasers, coatings have been produced which will generate a distinct coloured image on exposure to near, as well as mid or far-IR irradiation.
WO 2005/068207 describes an ink formulation comprising a marking component and a metal salt that absorbs laser irradiation at 700-2000 nm and thereby causes the marking component to change colour. This allows the utilization of diode and CO2 lasers for imaging applications on packaging, for example.
WO 2007/141522 describes an ink formulation comprising a marking component and a metal salt that absorbs laser irradiation at 780-2500 nm and thereby causes the marking component to change colour. A preferred metal salt is reduced indium tin oxide.
EP2361783 describes a thermosensitive recording medium including a support; an image recording layer on the support; and an inorganic material in particle form as a light-heat conversion material, wherein the inorganic material has a ratio of Y to X, represented by Y/X, of 2 or greater, where X denotes an average value of absorption intensities with respect to light having wavelengths in the range of 400 nm to 700 nm, and Y denotes a maximum value among absorption intensities with respect to light having wavelengths greater than 700 nm but smaller than or equal to 1,200 nm. The light-heat conversion material may include particles of at least one of a metal boride and a metal oxide and a tungsten oxide compound is mentioned as an option for the metal oxide. The disclosure focuses on the formation of a thermoreversible recording medium.
It is desirable to provide to provide further ink formulations having good laser imaging efficacy, but which have an essentially negligible impact on the background colour of the coating.